(1) Field of the Invention
The present invention relates to an mouse model for human menopause which provides a simulated comparison between early and late menopause. In particular the present invention uses time after an ovariectomy of the mouse to simulate early and late human menopause.
(2) Description of Related Art
Hormone replacement therapy (HRT) with estrogen in postmenopausal women is used to alleviate menopausal symptoms such as vasomotor and urogenital dysfunction, as well as to reduce the risk of bone fractures and cardiovascular disease, and it may also decrease the debilitating symptoms of Alzheimer""s disease (Stampfer, M. J., et al., N Engl J Med 325:756-762 (1991); Riggs, B. L., et al., N Engl J Med 327:620-627 (1992); Compston, J. E., Br Med Bull 48:309-344 (1992); Col, F. N., et al., JAMA 277:1140-1147 (1997); The writing group of PEPI trial, JAMA 276:1389-1396 (1996); Henderson, V. W., et al., Arch Neurol 51:896-900 (1994); and Ming-Xin, T., et al., The Lancet 348:429-432 (1996)). While most breast cancers occur in postmenopausal women, the role of menopause per se in the etiology of breast cancer is not known. However, there is solid epidemiological evidence for an important role of ovarian hormones in mammary cancer development (MacMahon, B., et al., J. Natl Cancer Inst 50:21-42 (1973)). Lifetime total exposure of the mammary gland to the mitogenic effects of ovarian hormones has been proposed to be a major risk factor for the development of breast cancer (MacMahon, B., et al., J. Natl Cancer Inst 50:21-42 (1973)). However, very little is known about the mitogenic effects of estrogen in the postmenopausal human breast.
The mouse has been widely studied in vivo for elucidating roles of hormones in mammary gland development and function, and the mouse mammary gland is similar to the human breast in many aspects of hormonal regulation of cell proliferation (Neville, M. C., et al., The mammary gland development, regulation and function. Plenum Press New York and London 625 (1987)). Most mammals, including mice, maintain their reproductive capacity throughout their entire life and do not experience natural menopause (Kirkwood, T. B. L., Comparative and evolutionary aspects of longevity In: Adelman R., et al. (eds) Handbook of the biology of aging. Van Nostrand Reinhold Co. New York 27-42 (1985)). However, a menopausal state in animals can be induced surgically, by ovariectomy. This is comparable to the situation in women who undergo bilateral ovariectomy prior to menopause, and prematurely experience the symptoms and side effects of menopause soon after surgery. In many instances women start hormone replacement therapy in the early postmenopausal period in order to alleviate menopausal symptoms. However, older postmenopausal women who have never previously received HRT are also given estrogen for its ability to reduce osteoporosis and decrease risk of cardiovascular disease (Michaelsson, K., et al., Br Med J 316:1858-1863 (1997); Prelevic, G. M., et al., Bailliere""s Clinical Endocrinology and Metabolism 11:311-340 (1997); Leveille, S. G., et al., J. Am Geriatr Soc 45:1496-1500 (1997); and Miller, K. L., Clin Obst Gynec 39:912-932 (1996)). Thus it was of interest to study the effects of estrogen in early vs. late postmenopause.
In order to counteract the negative side effects of menopause, such as vasomotor and urogenital dysfunction and to reduce osteoporosis and cardiovascular disease, women are prescribed hormone replacement therapy (HRT) with an estrogen (E) or estrogen plus progestin (P) (Hutchinson, T. A., et al., Lancet 2:705-709 (1979); Weiss, N. S., et al., N Engl J Med 303:1195-1198 (1980); Ross, R. K., et al., Lancet 1:858-860 (1981); Stampfer, M. J., et al., N Engl J Med 313:1044-1049 (1985); The writing group of PEPI trial. JAMA 276:1389-1396 (1996); Udoff, L., et al., Obstet Gynecol 86:306-316 (1995). Since the majority of postmenopausal women have an intact uterus, combination or sequential HRT with E and P are most commonly prescribed.
HRT also has negative consequences. Length of lifetime exposure to ovarian hormones has been proposed to be a major risk factor for the development of breast cancer (MacMahon, B., et al., J Natl Cancer Inst 50:21-42 (1973)). Furthermore, in contrast to that ability of P to inhibit E effects in the uterus, P synergizes with E to produce a maximal proliferative response in the mammary gland of the rodent and monkey (Haslam, S. Z., Endocrinology 122:464-470 (1988); Wang, S., et al., Endocrinology 127:2183-2189 (1990); Cline, J. M., et al., Am J Obstet Gynecol 174:93-100 (1996)). We have recently reported that normal breast tissues from postmenopausal women receiving combined HRT with estrogen plus progestin, exhibit significantly higher epithelial cell proliferation and greater epithelial density compared to no HRT or E alone HRT (Hofseth, L. J., et al., J Clin Endocrinol Metab 84:4559-4565 (1999)). Since HRT with E+P is associated with greater proliferation in the postmenopausal breast than E alone, this could have important implications for increased breast cancer risk. This is supported by a recent epidemiological study that found a greater risk of breast cancer in women who received E+P HRT than those who received E alone HRT (Magnusson, C., et al., Int J Cancer 81:339-344 (1999)).
Women usually initiate HRT in the early postmenopausal period in order to reduce menopausal symptoms. However, older postmenopausal women who have never previously received HRT now also receive HRT to reduce osteoporosis and cardiovascular disease. Thus, a new variable, the timing of initiation of HRT in early vs.late postmenopause, also requires evaluation for its effect on breast proliferation.
The murine mammary gland has been widely studied in vivo for elucidating the role of hormones in mammary gland development and function and is similar to the human breast in many aspects or hormonal regulation of cell proliferation (Neville, M. C., et al., The mammary gland development, regulation and function. Plenum Press, New York and London (1987)). Other related art is:
Iizuka, S., et al., Methods and Findings in Experimental and Clinical Pharmacology, V20, N1 (January-February), P39-46 (1998);
Yagi, K., ACTA Biochimica Polonica, V44, N4, P701-709 (1997);
Roux, C., et al., BONE, V19, N5 (November), P463-468 (1996);
Schiffenbauer, Yael S., et al., Proc. Natl. Acad. Sci. USA Vol. 94, pp 13203-13208 (1997);
Jagger, C. J., et al., J. Clin. Invest. Vol. 98, No. 10, 2351-2357 (1996);
Whelton, B. D., et al., Toxicology 119 141-153 (1997);
Wilson, Iain A., et al., European Journal of Pharmacology 381 93-99 (1999);
Whelton, B. D., et al., Toxicology 119 123-140 (1997);
Whelton, B. D., et al., Toxicology 119 103-121 (1997); and
Shimizu, H. et el., Journal of Endocrinology 154 285-292 (1997).
The present invention relates to a method for determining a response to a biologically active agent in lower laboratory test mammals which comprises:
(a) surgically ovariectomizing the lower mammal at a baseline time;
(b) dividing the ovariectomized lower mammals into at least two groups;
(c) treating a first of the groups of the ovariectomized mammals with the biologically active agent within a first period less than 21 days, from the baseline time;
(d) treating a second of the groups of the ovariectomized mammals with the biologically active agent with a second period of greater than about 35 days from the baseline time;
(e) comparing the results for the first and second groups of mammals to determine the response to biologically active agent.
In particular the present invention relates to mice as the test mammals. The invention approximates early and late menopause in humans.
The biologically active agent can be estrogen alone or in combination with progestin. These agents can be used alone or in combination with other agents which suppress or potentiate their activity. The method can also be used with other compounds which regulate biological activity in females including chemotherapeutic agents including SERMS as described in U.S. Pat. Nos. 5,994,370, 5,395,842, and 5,147,880.
The test mice can be normal or predisposed to a particular disease such as a tumor. All of this is well known to those skilled in the art.
The aim of the present invention is to particularly enable the identification of compounds which positively regulate the effects of estrogen and/or progesterone to prevent disease, such as breast tumors. It is particularly desirable to regulate negative results from estrogen in humans and the present invention enables the simulation of such regulation in a safe and effective manner.